1. Field of the invention
The resent invention relates to an antiskid braking system for preventing a skid of a car during braking.
2. Description of Related Art
In order to prevent wheels from locking due to excessive hydraulic braking pressure during braking so as to assure reliable steering and braking, there have been widely used various kinds of antiskid braking systems. Such an antiskid braking system cooperates with a control unit which causes an electronically controlled valve provided in a hydraulic braking pressure control system to, on one hand, reduce hydraulic braking pressure when wheels are on the verge of locking and, on the other hand, intensify the hydraulic braking pressure with a gradual growth in wheel speed resulting from a decline in the hydraulic braking pressure. The control unit calculates a slippage rate from a vehicle speed, which is represented by the greatest one of speeds of the wheels detected by, for instance, wheel speed sensors, respectively, and the wheel speeds and commences reducing hydraulic braking pressure at a time the slippage rate reaches a target slippage rate which is referred to as a threshold slippage rate for pressure reduction. When the control of hydraulic braking pressure in this manner, which is called an antiskid braking control, is continually performed until the car stops, the wheels are prevented from locking and, as a result, prevent the car from going into a skid, so as to assure the stability of steering and enable to bring the car to a halt in a short braking distance.
Much attention has been given to various approaches relating to braking techniques by means of antiskid braking systems which include the improving of the stability of steering and reducing braking distance. One such effort is that described in Japanese Unexamined Patent Publications Nos. 5-4574 and 6-56019. The antiskid braking system is to alter the pattern of operation of the antiskid braking system between high speed driving and low speed driving.
Incidentally, upon braking a car, a load shifts toward the front wheels. Considering that a car traveling at a high speed of, for instance, approximately 100 Km/h is braked and brought into a halt in a short time, the car stops with a large load shifted to and imparted to the front wheels, resulting in that the front wheels still receive reaction from a road surface even when the car looses speed. Accordingly, when a car equipped with an antiskid braking system enters into antiskid braking control during braking, the front wheels receive heavy reaction from the road surface even at a low vehicle speed in a range of speeds less than, for instance, 20 Km/h, the car causes a prompt revival of wheel speed even when negotiating hydraulic braking pressure reduction leniently in the low speed range.
On the other hand, when the car travelling at a moderate vehicle speed of, for instance, 50 Km/h on a road of the same surface conditions as above is brought into a halt, the reaction imparted to the front wheels is less heavy as compared with braking the car travelling at a high speed. This leads to a delayed revival of wheel speed unless negotiating hydraulic braking pressure reduction promptly in the low speed range.